Progress in understanding the intricate development of the human central nervous system and elucidating the mechanisms of neuropsychiatric disorders in patients has been greatly limited by restricted access to functional human brain tissue. While studies in rodents and other mammals have provided important insights into the fundamental principles of neural development, we know little about the cellular and molecular processes responsible for the massive expansion of the forebrain in primates, nor many of its human specific features. In recent years, a paradigm shift has been achieved in the field with the introduction of cellular reprogramming—a process during which terminally differentiated somatic cells can be converted into pluripotent stem cells, named human induced pluripotent stem cells (hiPSC). These hiPSCs can be generated from any individual and, importantly, can be directed to differentiate in vitro into all germ layer derivatives, including neural cells.
While the methods and efficiency of generating hiPSCs have been significantly improved and standardized across laboratories, the methods for deriving specific neuronal cell types and glial cells remain challenging. Over the past decade, improvements in neural specification and differentiation protocols of pluripotent stem cells in monolayer have led to the generation of a variety of cell types. Nonetheless, two-dimensional (2D) methods are unlikely to recapitulate the cyto-architecture of the developing three-dimensional (3D) nervous system or the complexity and functionality of in vivo neural networks and circuits. Moreover, these methods are laborious, costly, have limited efficiency, give rise to relatively immature neurons and incompatible with long-term culturing of neurons.
Pharmaceutical drug discovery utilizes the identification and validation of therapeutic targets, as well as the identification and optimization of lead compounds. The explosion in numbers of potential new targets and chemical entities resulting from genomics and combinatorial chemistry approaches over the past few years has placed massive pressure on screening programs. The rewards for identification of a useful drug are enormous, but the percentages of hits from any screening program are generally very low. Desirable compound screening methods solve this problem by both allowing for a high-throughput so that many individual compounds can be tested; and by providing biologically relevant information so that there is a good correlation between the information generated by the screening assay and the pharmaceutical effectiveness of the compound.
Some of the more important features for pharmaceutical effectiveness are specificity for the targeted cell or disease, a lack of toxicity at relevant dosages, and specific activity of the compound against its molecular target. Therefore, one would like to have a method for screening compounds or libraries of compounds that allows simultaneous evaluation for the effect of a compound on the biologically relevant cell population, where the assay predicts clinical effectiveness.
The effect of drugs on specific human neural or glial cell types is of particular interest, where efficacy and toxicity may rest in sophisticated analysis of cell migration, activity, or the ability of neurons to form functional networks, rather than on simple viability assays. The discrepancy between the number of lead compounds in clinical development and approved drugs may partially be a result of the methods used to generate the leads and highlights the need for new technology to obtain more detailed and physiologically relevant information on cellular processes in normal and diseased states.
A number of important clinical conditions are associated with altered neuronal or glial function, including neurodegenerative disorders (Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS), or psychiatric conditions such as schizophrenia and other psychoses, bipolar disorders, mood disorders, intellectual disability (ID) or autism spectrum disorders (ASD).
In addition to pharmaceutical drug discovery, there is a pressing need for meaningful screening platforms to identify and explore specific toxicity effects due to the increasing number of new therapeutic compounds and chemical substances with human exposure. Particularly, in the field of neurotoxicity, assays capable of assessing the impairment of neuronal or glial function are still lacking for human cells.
Therefore, the development of in vitro screening platforms that recapitulate highly functional human tissue is of utmost importance.
Publications. Methods to reprogram primate somatic cells to a pluripotent state include differentiated somatic cell nuclear transfer, differentiated somatic cell fusion with pluripotent stem cells and direct reprogramming to produce induced pluripotent stem cells (iPS cells) (Takahashi K, et al. (2007) Cell 131:861-872; Park I H, et al. (2008) Nature 451:141-146; Yu J, et al. (2007) Science 318:1917-1920; Kim D, et al. (2009) Cell Stem Cell 4:472-476; Soldner F, et al. (2009) Cell. 136:964-977; Huangfu D, et al. (2008) Nature Biotechnology 26:1269-1275; Li W, et al. (2009) Cell Stem Cell 4:16-19).